Diagnostic tests frequently require separation of a patient's whole blood sample into components, especially cellular portions from non-cellular portions e.g. serum or plasma from cells. For instance, plasma is obtained from anticoagulated blood and still contains all of the coagulation proteins, whereas serum is obtained from clotted blood with the bulk of the coagulation proteins being retained with the clot and red blood cells. Samples of whole blood are typically collected by ventipuncture through a special cannula or needle attached to a syringe or an evacuated collection tube. The sample of blood in the form that is to be separated into components is typically drawn, using a needle, through a penetrable self-sealing elastomeric closure or other stopper into an evacuated tube. Separation is then accomplished e.g. by rotation of the tube in a conventional centrifuge e.g. a swinging bucket or a fixed angle centrifuge, as the different components of the whole blood have different densities, as described in U.S. Pat. No. 4,152,269 of A. L. Babson.
It is frequently desirable to physically isolate the separated phases from each other, so that separation of the phases is maintained after centrifugal rotation has ceased. Isolation may be accomplished by interposing a gel material between the phases, the gel material typically being a silicone that is placed in the tube at the time of manufacture. These gels have densities that are intermediate those of the phases being separated and become interposed between the phases during centrifugal rotation, as is described in U.S. Pat. No. 4,350,593 of Kessler and U.S. Pat. Nos. 3,852,194 and 4,083,784 of A. R. Zine. However, the gels may contain absorbed substances that can interfere with blood analyses or adsorb specific compounds from the blood e.g. tricyclic drugs, the separation of plasma or serum from blood cells may be incomplete and severe jarring or shaking e.g. as in shipping of samples, may disrupt the seal and result in interaction of the separated phases.
U.S. Pat. No. 3,929,646 of S. L. Adler describes a serum separator for use in a centrifuge separation system. The separator is initially positioned at one end of a tube, adjacent to a tube stopper, with the whole blood sample being contained within the tube. When the tube is subjected to centrifugal force, the separator moves away from the stopper and towards the other end of the tube. The separator is designed so as to have a density that is between the densities of two phases of blood, so that when centrifugation is complete, it is positioned between the two phases. The separator has openings to allow the lighter phase of the blood to pass through the separator and allow the separator to move through the blood sample. Thus, a blood sample can be separated into plasma and cell phases that are physically separated. However, the openings may become plugged with clotted blood fractions or conversely permit migration between phases when there is no centrifugal force; in either event effective separation will be lost.
An apparatus and method of separating blood phases by rotation of a tube about its longitudinal axis i.e. axial rotation, are described in U.S. Pat. No. 4,828,716 of J. A. McEwen et al. The blood sample is introduced to the tube through a cap assembly that consists of a pierceable closure and a separator that has a one-way valve. The tube is then rotated about its longitudinal axis; the heavier cellular phase lines the tube wall and thereby separates from the lighter non-cellular (plasma or serum) phase. Once separation has been achieved, an axial probe penetrates the pierceable closure, detaches the separator from the closure and forces the separator down the tube. The axially-located non-cellular phase passes through the separator. An optical sensor is utilized to detect when the cellular phase begins passing into the separator, and to stop movement of the separator. Thus, the two phases are physically separated. However, it is believed that a separator that is more reliable in operation and which may be manufactured in a cost effective manner is required.
A related application of R. P. Luoma filed concurrently herewith is directed to so-called double ended tubes.